The present invention relates generally to signal processing, and more specifically to techniques for suppressing acoustic echo due to serious nonlinearity.
Full-duplex hands-free communication systems are commonly used for many applications, such as speakerphone, hands-free car kit, teleconferencing system, cellular phone, and so on. For each of these systems, one or more microphones in the system are used to pick up an acoustic signal emitted by a speaking user, which is then processed and transmitted to a remote user. However, the microphones may also pick up undesirable reflections of the acoustic signal from the borders of an enclosure, such as a room or a car compartment. The propagation paths for the reflections may change due to various factors such as, for example, movement of the microphones, the loudspeaker, and/or the speaking user, volume changes on the loudspeaker, and environment changes. As a result, the electro-acoustic circuit in the system may become unstable and produce howling, which is highly undesirable.
In the case of a telecommunication system, a speech signal from a remote speaking user is outputted from a loudspeaker, and portions of this speech signal may be reflected to the microphones and transmitted back to the remote user. This acoustic disturbance is referred to as echo. Echo may also be caused by signal reflections generated by a hybrid circuit that converts between 4-wire circuit and 2-wire circuit. Regardless of the cause of echo, users are generally annoyed of hearing their own voice delayed, for example, by the path of the system.
Echo suppression or cancellation is often required in many communication systems to suppress or eliminate echo as well as to avoid howling effects. For example, echo cancellation is typically used in full-duplex communication environments where the speaker and microphone may be located some distance away from a user. Examples of such environments include hands-free speakerphone (e.g., in a vehicle or a room), Internet/Intranet Protocol phone, and so on.
Conventionally, echo cancellation is achieved by a circuit that employs an adaptive filter. The adaptive filter performs echo cancellation by deriving an estimate of the echo based on a reference signal, which may be a line output from a communication or telematics device such as a cellular phone or some other device. The adaptive filter is typically able to remove the portion of the echo that is correlated to the reference signal.
However, conventional echo cancellation techniques are not able to remove certain portions of the echo. For example, nonlinearity of the circuitry in the system (e.g., the speaker, analog-to-digital (A/D) converter, digital-to-analog (D/A) converter, and so on) generates echo that is not correlated to the reference signal. This type of echo cannot be canceled by conventional echo cancellation techniques that employ only an adaptive filter. Moreover, user movement, position changes in the microphones and loudspeakers, and volume changes can cause the echo path to vary. This results in time-varying echo that typically cannot be canceled very well, particularly if the echo path changes faster than the convergence rate of the adaptive filter.
Nonlinear echo cancellation techniques may be used to attempt to cancel the residual echo that is not canceled by the adaptive filter in the echo canceller. However, these techniques typically cannot cancel echo due to serious nonlinearity. Nonlinear echo may be caused by various conditions such as an overdriven speaker, a microphone in saturation, mechanical vibration, and so on. These techniques also cannot handle high volume echo. Moreover, some conventional nonlinear echo cancellation techniques, such as a center clipper, can cause voice distortion by cutting off low power voice signal. Conventional center clippers are described, for example, in U.S. Pat. Nos. 4,031,338, 4,679,230 and 5,475,731, and European Patent Nos. EP-0164159-A1 and EP-0164159-B1. Other conventional nonlinear echo cancellation techniques, such as conventional post filters, also cannot deal with large echo and serious nonlinearity.
As can be seen, techniques that can effectively cancel large echo and/or echo due to serious nonlinearity in communication systems are highly desirable.